Plant oils are used in a variety of applications. Novel vegetable oil compositions and improved approaches to obtain oil compositions, from biosynthetic or natural plant sources, are needed. Depending upon the intended oil use, various different fatty acid compositions are desired. Plants, especially species which synthesize large amounts of oils in seeds, are an important source of oils both for edible and industrial uses.
Oleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable sources. It is considered one of the healthier sources of fat in the diet and is commonly used as a replacement for fat sources that are high in saturated fats.
Diets in which fat consumption are high in oleic acid have been shown to reduce overall levels of cholesterol, arteriosclerosis and cardiovascular disease. Specifically, oleic acid has been shown to raise levels of high-density lipoproteins (HDLs) known as “good cholesterol”, while lowering low-density lipoproteins (LDLs) also known as the “bad” cholesterol. Thus, the development of new and inexpensive sources of foods comprising healthier forms of fatty acid is desirable.
Plants synthesize fatty acids via a common metabolic pathway known as the fatty acid synthetase (FAS) pathway. Beta-ketoacyl-ACP (acyl carrier protein moiety) synthases are important rate-limiting enzymes in the FAS of plant cells and exist in several versions. Beta-ketoacyl-ACP synthase I catalyzes chain elongation to palmitoyl-ACP (C16:0), whereas Beta-ketoacyl-ACP synthase II catalyzes chain elongation to stearoyl-ACP (C18:0). Beta-ketoacyl-ACP synthase IV is a variant of Beta-ketoacyl-ACP synthase II, and can also catalyze chain elongation to 18:0-ACP. In soybeans, the major products of FAS are 16:0-ACP and 18:0-ACP. The desaturation of 18:0-ACP to form 18:1-ACP is catalyzed by a plastid-localized soluble delta-9 desaturase (also referred to as “stearoyl-ACP desaturase”).
The products of the plastidial FAS and delta-9 desaturase, 16:0-ACP, 18:0-ACP, and 18:1-ACP, are hydrolyzed by specific thioesterases (FAT). Plant thioesterases can be classified into two gene families based on sequence homology and substrate preference. The first family, FATA, includes long chain acyl-ACP thioesterases having activity primarily on 18:1-ACP. Enzymes of the second family, FATB, commonly utilize 16:0-ACP (palmitoyl-ACP), 18:0-ACP (stearoyl-ACP), and 18:1-ACP (oleoyl-ACP). Such thioesterases have an important role in determining chain length during de novo fatty acid biosynthesis in plants, and thus these enzymes are useful in the provision of various modifications of fatty acyl compositions, particularly with respect to the relative proportions of various fatty acyl groups that are present in seed storage oils.
The products of the FATA and FATB reactions, the free fatty acids, leave the plastids and are converted to their respective acyl-CoA esters. Acyl-CoAs are substrates for the lipid-biosynthesis pathway (Kennedy Pathway), which is located in the endoplasmic reticulum (ER). This pathway is responsible for membrane lipid formation as well as the biosynthesis of triacylglycerols, which constitute the seed oil. In the ER there are additional membrane-bound desaturases, which can further desaturate 18:1 to polyunsaturated fatty acids.
The soybean genome possesses two seed-specific isoforms of a delta-12 desaturase FAD2, designated FAD2-1A and FAD2-1B, which differ at only 24 amino acid residues. The genes encoding FAD2-1A and FAD2-1B are designated Glyma 10g42470 on Linkage Group 0 and Glyma 20g24530 on Linkage Group I on the soybean genome sequence, respectively (Glyma1.0, Soybean Genome Project, DoE Joint Genome Institute). FAD2-1A and FAD2-1B are found in the ER where they can further desaturate oleic acid to polyunsaturated fatty acids. The delta-12 desaturase catalyzes the insertion of a double bond into oleic acid (18:1), forming linoleic acid (18:2) which results in a consequent reduction of oleic acid levels. A delta-15 desaturase (FADS) catalyzes the insertion of a double bond into linoleic acid (18:2), forming linolenic acid (18:3).
TABLE 1Characteristics of the major Fatty AcidsCarbons:Double BondsNameSaturation16:0Palmitic AcidSaturated18:0Stearic AcidSaturated18:1Oleic Acidmonounsaturated18:2Linoleic Acidω-6 polyunsaturated18:3α-Linolenic Acidω-3 polyunsaturated
The designations (18:2), (18:1), (18:3), etc., refer to the number of carbon atoms in the fatty acid chain and the number of double bonds therein, Table 1. As used herein, the designations sometimes take the place of the corresponding fatty acid common name. For example, oleic acid (18:1) contains 18 carbon atoms and 1 double bond, and is sometimes referred to as simply “18:1”.
While previous research has demonstrated the important role of the FAD2-1A gene for increasing oleic acid, no reports have demonstrated a direct effect of the FAD2-1B gene on oleic acid accumulation. Soybean is a commodity crop that provides a major component of the fats and oils in the American diet. Soybean is considered an oilseed, and it typically contains about 20% oleic acid as part of the fatty acid profile in the seed oil.
Soybean oil is used by the food industry in a variety of food products including cooking oils, salad dressings, sandwich spreads, margarine, bread, mayonnaise, non-dairy coffee creamers and snack foods. Soybean oil is also used in industrial markets such as biodiesel and biolube markets.
For many oil applications, low saturated fatty acid levels are desirable. Saturated fatty acids have high melting points which are undesirable in many applications. When used as a feedstock or fuel, saturated fatty acids cause clouding at low temperatures, and confer poor cold flow properties such as pour points and cold filter plugging points to the fuel. Oil products containing low saturated fatty acid levels may be preferred by consumers and the food industry because they are perceived as healthier and/or may be labeled as “low in saturated fat” in accordance with FDA guidelines. In addition, low saturate oils reduce or eliminate the need to winterize the oil for food applications such as salad oils. In biodiesel and lubricant applications, oils with low saturated fatty acid levels confer improved cold flow properties and do not cloud at low temperatures.
Various technologies for generating mid to high oleic acid levels in soybean plants are known. For example, U.S. Patent Publication No. 2007/0214516 discloses a method for obtaining soybean plants that have moderately increased levels of oleic acid. However, this technology requires the genetic modification of soybean plants through the introduction of a transgene by transgenesis.
While transgenic soybean lines have been generated that produce soybean oil containing mid to high levels of oleic acid, non-genetically modified (non-GMO) soybean plant lines that produce seed with mid to high oleic acid content is desirable.